<script type="text/javascript">
function viewblogs(url)
{
var popupStyle = "dialogheight=300px;dialogwidth=450px;dialogleft:200px;dialogtop:200px;status:no;help:no;";
var var1 = window.showModalDialog(url,this, '', popupStyle);
}
</script>
call this in code behind
string pageurl = "ViewBlogs.aspx";
Page.ClientScript.RegisterStartupScript(this.GetType(), "aa", "<script language='javascript'>viewblogs('" + pageurl + "');</script>");
still have any doubt reply me